Why are lens and format measurements so absurd?

[Guest mc_k]

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Though when the question is about the percentage the frames for the two versions of the M8 differ, we can see, that this percentage is not related to the lens, but to the calibration of the frame for a certain focal length.

 

I think the figures show just what you would expect, the maximum error increases with the focal length and decreases with the reference distance.

 

Thanks for the nice summary by the way--I didn't know why they kept changing the distances.

[ho_co]

A bit off-topic now, but to return to the original question: One reason to keep the reference to another format is that the crop factor also gives a guide to depth of field. (See article "Form follows format," LFI 3/2006, pp 40-47.)

 

That is, my 50/2 lens on a camera such as the M8 with a crop factor of 4/3 relative to 35mm becomes the equivalent of a 67mm f/3.3 in regard to both angle of view and depth of field.

 

Referring only to field-of-view would mean a loss of information.

[Wiggi]

Why do we continue to use absurd focal-length measurements in photography? 35mm-equivalent, 90mm-equivalent, 200mm-equivalent and so on? Actual focal lengths -- 5mm-15mm, even 24mm or 35mm -- are meaningless if you don't know the sensor size. But multiplying by, for example, a 1.25 crop factor to convert to 35mm equivalent just adds another layer of complexity. Zooms have 3X, 5X, 10X and more X. To the uninitiated, the more X the better, but there is seldom any mention of the focal lengths or what they mean.

 

I am aware this idea is not new, but especially given the variety of sensor sizes now in use, why not use angle of view in degrees? Thus,

 

92º = 21mm (35mm film or "full frame" sensor size)

84º = 24mm

63º = 35mm

45º = 50mm

27º = 90mm

12º = 200mm

 

..and so forth. Or even better, just use round numbers, 90º, 20º ... A logical, universal, and meaningful set of numbers, surely!

 

We all know the charakter of a lens when you say the focal length. When you say to me the angle of view is 84°, I must think a lttle before I know you mean a 24mm lens.

 

It is like Horsepower and Kilowatt of cars. In Germany since nearly 35 years the power must be said in Kilowatt. When you say the car has for example 250 kW, nobody knows what you mean. So the car manufacturers can also write the Horsepower in the description. Everyone knows, what 340 hp mean.

 

And so it is with the focal length. Maybe it would help, when you write the angle of view with the focal length on the lens or camera.

[NZDavid]

We all know the charakter of a lens when you say the focal length.

 

Sorry, no we don't. (I agree about horsepower, though; expressing a car engine's power in kW is like describing it as a giant electric heater.)

 

But my original post was not limited only to Leica M cameras. As I said earlier, how about new photographers? Or photographers using one of the myriad "new" digital formats? For people like my friend and her Nikon superzooom," 5-75mm" and "15X" (the only measurements stated) don't mean a lot except quite wide angle and telephoto. Angles of view in degrees would be more useful.

 

Getting to grips with three main formats in the film era (35mm, medium format, maybe large format) was not too hard; there is lots more choice now. There are more sensor sizes than ever, as described above: several fingernail size ones, 1/1.17", APS-C, APS-H, "full frame" (24x36mm), "medium format" and so on. There is, as yet, no standard size sensor for digital cameras. Even if a standard size is agreed upon, at some stage, it is unlikely to be the same as 35mm film. Why then, base all measurements on the 35mm system? And why convert non 35mm focal-lengths to 35mm equivalent?

 

Focal-length (s) may remain constant, but few people swap lenses between cameras with different size sensors. The M8-M9, perhaps a few SLR system lenses, would be some of the very few.

 

To put it another way, if you reach for lens, what are the most important things about it you need to know? I would say, 1) angle of view (and when you say, for example, 24mm, you are really making a rough calculation in your head how much of the scene it will cover, and 2) speed.

 

To help visualize angle of view, I came across this useful guide -- a real digital guide:

 

Digital Lens Computer

[pgk]

Why then, base all measurements on the 35mm system?

I think that to a Medium Format film shooter he/she will think in terms of 6x6cm focal lengths - ie 80mm standard focal length - rather than consider lenses in 35mm terms.

 

The problem with using angle of view is that it is a variable - depending on format. Focal length is (generally) not, or is at least less so. However to take your argument further look at many of the new internal focus macro lenses which actually use a 'shifting focal length' to enable them to use moving internal groups of elements to focus closer - these are neither fixed focal lenses nor zooms in the traditional sense as they do not focus to infinity at shifted focal length. They are however still given a designated focal length (I assume based on their focal length when operated at infinity setting) by their manufacturers. I wouldn't be surprised if we finally do see real macro zooms in which actual focal length shifts are extremely complex (perhaps only revealed in exif data?).

 

As you say, in the past things were simpler with relatively fixed formats and many fixed focal lenses. Today its far more complex and accordingly there are fewer simple answers.

[pop]

I think there's a bit of a contradiction or non-sequitur here.

 

New users who are not accustomed to numbers characterizing lenses do - by definition - not have a preference as to how to number their lens or lenses.

 

Current users who have learned what the number does, on the other hand, do rely on those numbers being visible.

 

So why change a perfectly reasonable and useful convention if it takes away an important property from those in the know while just giving one kind instead of another kind of numbering scheme for those who don't care?

[NZDavid]

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It's not just me wondering about angles of view. I remember reading about this issue yonks ago. In the current (October 2010) issue of Popular Photography, Dan Richards talks about the proliferation of sensors and their effect on lenses:

 

"So, with smaller and smaller sensors, you need progressively wider-angle lenses to maintain the full-frame equivalent.

 

"Confusing? A better way would be to state the angle of view lenses project on a given format, which would require no translation into an “equivalent.” An ultrawide would give you a greater than 85-degree angle of view; a “normal” lens, around 40 degrees. Superteles would be those that give you single-digit angles of view. Unfortunately, nobody describes them that way."

 

More here: Why Sensor Size Changes the Angle of View | Photography - PopPhoto.com Offers Camera Reviews and Exclusive Photo Tips

 

And in this website on panoramic photography, the author agrees:

 

"With the advent of Digital photography came a variety of sensor sizes and formats resulting in the focal length of the lens becoming a meaningless number when considering its Angle of View, unless the sensor dimensions are known. The camera manufacturers, retailers and magazines adopted the convention of quoting the 35mm equivalent so that those familiar with the lenses used for 35mm Film cameras could relate to the Angle of View (e.g. focal length 5.2-26.0mm (29-145mm - 35mm equivalent). This is fine for those who remember 35mm Film photography! I feel however that a golden opportunity to adopt the convention of expressing what the lens sees as its Angle of View was missed. Quoting an Angle of View for a lens for a particular camera would mean that the knowledge of the lens focal length and sensor dimensions would not be necessary in order to understand what the lens sees."

 

See: Lens Angle of View

 

If some manufacturer brings in angles of view as a measurement sometime soon, remember you read about it here first!

[pop]

"So, with smaller and smaller sensors, you need progressively wider-angle lenses to maintain the full-frame equivalent. (...)"

 

That seems like a deliberately misleading statement to me. If a lens produces the same field of view as a 50mm lens for the 35mm film format does, it is by definition not a wider-angle lens.

 

The camera manufacturers, retailers and magazines adopted the convention of quoting the 35mm equivalent so that those familiar with the lenses used for 35mm Film cameras could relate to the Angle of View ... This is fine for those who remember 35mm Film photography! (...) Quoting an Angle of View for a lens for a particular camera would mean that the knowledge of the lens focal length and sensor dimensions would not be necessary in order to understand what the lens sees."

 

It is even possible that camera manufacturers have spent some thought on that matter. For time immemorial, cameras have been supplied with data which is meaningful to the prospective user.

 

If camera manufacturers had found that users wanted the angle of view for their purchasing decisions, they would have published the angle of view. Don't tell me that camera manufacturers are weak on marketing.

 

Is there an angle of view on the Kodak Brownie? On the Instamatic? On a 110-type camera? On a PolaroidSX70?

 

If some manufacturer brings in angles of view as a measurement sometime soon, remember you read about it here first!

 

No, I didn't.

[elansprint72]

With my continental European limits I can only support your demand for an universal measurement system. Napoleon was not wrong in this case.

 

Though we have to accept, that there are a few people around, who like to stick on their inches and feet etc. So many descriptions by measurements have to be doubled which makes it very hard to remember all of them (I keep mixing up 1/4" and 3/8" for the tripod screws...). I am happy that photography is rather independant of Fahrenheit an Celsius; though we have Kelvin as well. Therefore many descriptions try to avoid this by using a sort of "brand" name: FF, APS, Micro 4/3 etc.

 

You are clearly not a plumber in France, where pipe-fittings use the British Standard Pipe threads, both straight and taper. Now that is illogical!

 

btw, I have been a committed Napoleonist since schooldays.

[luigi bertolotti]

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If some manufacturer brings in angles of view as a measurement sometime soon, remember you read about it here first!

 

Well, very often the angles of view are indeed detailed by manufacturers : simply they are written in the spec sheets that go with the camera, in the commercial brochures, in the specs published on the Net...

They aren't engraved on the lens mount, but this, imho, for the reason I wrote in my previous post (most of "lens-conscious" customers belong to a generation mentally tied and accustomed to 35 mm format & focals).

[Guest mc_k]

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I realize angle of view changes (slightly) from near focus to infinity..."a lens set at any distance greater than its minimum focusing distance will always record a little more of the subject within the full 24 x 36 mm format on the film than outlined by its frame in the viewfinder." -- Günter Osterloh...

 

I don't think the difference is so "slight" with a longer focal length. For example, I think the view at close focus of a 75 is closer to the nominal angle of view of a 90, than it is to a 75. Another reason why engraving the angle of view instead of the focal length is bunk.

[giordano]

I don't think the difference is so "slight" with a longer focal length. For example, I think the view at close focus of a 75 is closer to the nominal angle of view of a 90, than it is to a 75.

 

According to my spreadsheet:

 

Focal length Subject to sensor Angle (diagonal)

90 1000m 27.0

90 1000mm 24.4

75 1000m 32.2

75 700mm 28.4

Another reason why engraving the angle of view instead of the focal length is bunk.

 

I wouldn't say bunk ,but certainly it needs to be hedged with qualifications.

[Guest mc_k]

According to my spreadsheet:

 

Focal length Subject to sensor Angle (diagonal)

90 1000m 27.0

90 1000mm 24.4

75 1000m 32.2

75 700mm 28.4

 

 

...

 

Thanks for this. Looks like the 75mm close view (28.4 deg.) is closer to the 90mm angle of view (27 deg.) than the 75mm angle of view (32.2 deg.).

 

Do you know what expression the spreadsheet uses to calculate this?

[giordano]

Thanks for this. Looks like the 75mm close view (28.4 deg.) is closer to the 90mm angle of view (27 deg.) than the 75mm angle of view (32.2 deg.).

 

Do you know what expression the spreadsheet uses to calculate this?

 

It's =DEGREES(2*ATAN(43.2662/(2*D5))) where D5 contains u (the distance from the "centre" of the lens to the sensor). Obviously that's for the angle across the diagonal of the FF sensor.

 

For the "distant" ones I set v to 1km. Close up, v was 900mm for the 90mm lens and 616mm for the 75, giving u+v 1000mm and 701mm respectively.

 

Remember that if either lens has floating elements its focal length may change as you turn the focusing ring.

[Guest mc_k]

ah, I should have known you'd do it exactly.

 

...Thanks.

[Guest mc_k]

...

FOCAL LENGTH, REFERENCE DISTANCE, MAX ERROR

 

(from M8 manual)

28mm, 0.7m, 9%

90mm, 1m, 23%

 

(from M8.2 manual)

24mm, 2m, 7.3%

90mm, 2m, 18%

 

(from M9 manual)

28mm, 1m, 7.3%

135mm, 1m, 18%

...

 

I get actually 8.90%, 23.46%, 2.47%, 10.16%, 6.03%, and 42.02% resp. (from the lens equation, and taking the object distance from the film plane). As you can see this gibes with the M8 manual, but the rest of the figures are way off...so I could be wrong, or the type of lens may come into play, or Leica may have typos, etc. The last four Leica numbers look a little suspicious.

 

One other comment: in the textbook case, almost all of the error accumulates in the first 5m.

 

Once again ... this info is for "entertainment purposes only" ...

[NZDavid]

For anyone still interested in reading further, and for the sake of a comprehensive database on this topic, technology writer David Pogue in the New York Times is also dismissive of the continued use of 35mm-equivalent for all kinds of lenses, no matter the format or actual focal length. He suggests footballs. And there's lots more comment:

 

Camera Lens Focal Lengths: Measure for Measure - NYTimes.com

 

Of course, I know exactly what focal-length my Leica lenses are, but I'm still sticking to my guns (measured in either inches or mm).

 

Cheers,

David

[pop]

... technology writer David Pogue in the New York Times is also dismissive of the continued use of 35mm-equivalent for all kinds of lenses, no matter the format or actual focal length. He suggests footballs.

 

Maybe we take a standard-size item, like a football 10 feet away from you, and represent how big it appears in the frame.

 

Yes, fine. Obviously, every user knows the diameters or lengths of the sides of all his or her sensors. Given that the sensor size is known, anyway, every user can easily deduct the "strength" of his lens.

[Nicoleica]

For anyone still interested in reading further, and for the sake of a comprehensive database on this topic, technology writer David Pogue in the New York Times is also dismissive of the continued use of 35mm-equivalent for all kinds of lenses, no matter the format or actual focal length. He suggests footballs. And there's lots more comment:

 

Camera Lens Focal Lengths: Measure for Measure - NYTimes.com

 

Of course, I know exactly what focal-length my Leica lenses are, but I'm still sticking to my guns (measured in either inches or mm).

 

Cheers,

David

 

Would those be real footballs, or those funny shaped american things?

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Hallo,

Look here: Interesting Leica gear

[NZDavid]

Real footballs are oval rugby balls!